1. Field of the Invention
The present invention relates to a package for housing a photosemiconductor element.
2. Description of the Related Art
Hitherto, a package for housing a photosemiconductor element comprises a metallic substrate generally made of metal such as an iron-nickel-cobalt alloy and a copper-tungsten alloy, having a mounting portion for mounting the photosemiconductor element at the center of the top surface thereof, around the mounting portion a plurality of external lead terminals being fixed so as to penetrate through an insulating member from the top surface to the bottom surface of the substrate; a frame made of an iron-nickel-cobalt alloy and the like, the frame being bonded on the metallic substrate via a brazing material such as silver brazing material so as to surround the mounting portion for mounting the photosemiconductor element, the frame having a through hole on a side thereof; a cylindrical fixing member made of metal such as an iron-nickel-cobalt alloy, the cylindrical fixing member being attached around the through hole of the frame via a brazing material such as a gold-tin alloy and having a space therein in which an optical signal is transmitted; a light transmitting member made of amorphous glass and the like, the light transmitting member being attached to the cylindrical fixing member via a low melting point brazing material such as a gold-tin alloy having a melting point of 300 to 400xc2x0 C. to clog the inside of the fixing member; and a lid attached onto the top surface of the frame to hermetically seal the photosemiconductor element. The package for housing a photosemiconductor element is made into a photosemiconductor device as a product in the steps of bonding and fixing the photosemiconductor element to the mounting portion of the metal substrate, electrically connecting respective electrodes of the photosemiconductor element with external lead terminals via bonding wires, bonding the lid on the top surface of the frame, housing the photosemiconductor element hermetically within the housing composed of the metal substrate, the frame and the lid, and connecting an optical fiber to the cylindrical fixing member.
Such a photosemiconductor device functions as a photosemiconductor device for use in high speed optical communications and others in such a manner that the photosemiconductor element is optically excited by a driving signal supplied from an external electrical circuit, and the excited light is received by the optical fiber through the light transmitting member to be transmitted within the optical fiber.
However, the prior art package for housing a photosemiconductor element, which comprises a light transmitting member made of amorphous glass having a coefficient of thermal expansion of 8.5xc3x9710xe2x88x926/xc2x0C. (30 to 400xc2x0 C.) and a cylindrical fixing member made of an iron-nickel-cobalt alloy having a coefficient of thermal expansion of about 4.5 xc3x9710xe2x88x926/xc2x0C. (room temperature to 400xc2x0 C.) has a drawback that when the light transmitting member is attached to the cylindrical fixing member via a low melting point brazing material, thermal stress is caused between the light transmitting member and the cylindrical fixing member due to the difference in coefficient of thermal expansion between the two, and which thermal stress remains in the light transmitting member. Accordingly, when the light exited by the photosemiconductor element is passed through the light transmitting member to be received by the optical fiber, double-refraction of the excited light is caused due to the thermal stress existing in the light transmitting member, and consequently the optical fiber receives only part of the excited light, with the result that the light receiving efficiency of the optical fiber as well as the transmission efficiency of optical signal decreases.
Hence, in order to eliminate the drawback, it is conceivable to form a fixing member to which the light transmitting member made of the amorphous glass is attached, out of an alloy of from 40 to 60 wt % iron and from 40 to 60 wt % nickel, having a coefficient of thermal expansion close to that of the light transmitting member.
In the case where such a fixing member is formed out of the alloy of from 40 to 60 wt % iron and from 40 to 60 wt % nickel, since the coefficient of thermal expansion of the fixing member is about 9.5xc3x9710xe2x88x926/xc2x0C. (room temperature to 400xc2x0 C.), close to the coefficient of thermal expansion of the light transmitting member made of the amorphous glass (about 8.5xc3x9710xe2x88x926/xc2x0C.:30 to 400xc2x0 C.), in attaching the light transmitting member to the fixing member almost no thermal stress due to a difference in coefficient of thermal expansion occurs and accordingly there exists almost no thermal stress in the light transmitting member.
However, although no thermal stress is caused between the fixing member and the light transmitting member in attaching the light transmitting member to the fixing member, the frame is made of a metallic material such as an iron-nickel-cobalt alloy, having a thermal expansion of 4.5xc3x9710xe2x88x926/xc2x0C. (room temperature to 400xc2x0 C.), which is different from the coefficient of thermal expansion of the fixing member (about 9.5xc3x9710xe2x88x926/xc2x0C.: room temperature to 400xc2x0 C.) made of the alloy of from 40 to 60 wt % iron and from 40 to 60 wt % nickel, thermal stress due to the difference in coefficient of thermal expansion between the two occurs in attaching the fixing member on the outer surface around the through hole of the frame via a brazing material of a gold-tin alloy or the like. The thermal stress adversely affects the light transmitting member attached to the fixing member, namely causes double-refraction of the light excited by the photosemiconductor element in passing through the light transmitting member, which results in a drawback that the light receiving efficiency of optical fiber and the transmission efficiency of optical signal decrease.
The present invention has been devised in view of the drawbacks mentioned above and the object of the invention is to provide a package for housing a photosemiconductor element capable of causing excited light to be efficiently received by an optical fiber and achieving an enhanced transmission efficiency of optical signal.
The invention provides a package for housing a photosemiconductor element comprising:
a substrate having a mounting portion on a top surface of which a photosemiconductor element is mounted;
a frame attached onto the substrate so as to surround the mounting portion, having a through hole on a side thereof;
a cylindrical fixing member attached around the through hole of the frame, having a space therein in which an optical signal is transmitted;
a light transmitting member attached to the cylindrical fixing member, for clogging an inside of the fixing member; and
a lid attached to a top surface of the frame, for hermetically sealing the photosemiconductor element, wherein the fixing member is made of an alloy of from 40 to 60 wt % iron and from 40 to 60 wt % nickel.
According to the package for housing a photosemiconductor element of the invention, because the fixing member is made of the alloy of from 40 to 60 wt % iron and from 40 to 60 wt % nickel, the coefficient of thermal expansion of the fixing member is about 9.5xc3x9710xe2x88x926/xc2x0C. (room temperature to 400xc2x0 C.). Thereby, the coefficient of thermal expansion of the fixing member becomes close to 8.6xc3x9710xe2x88x926/xc2x0C. (30 to 400xc2x0 C.), which is the coefficient of thermal expansion of the light transmitting member. As a result, in attaching the light transmitting member to the fixing member almost no thermal stress due to a difference in coefficient of thermal expansion occurs between the two and almost no thermal stress exists in the light transmitting member. Accordingly, in transmitting excited light of the photosemiconductor element to the optical fiber member via the light transmitting member, the excited light is efficiently received by the optical fiber member as it is without being double-refracted in the light transmitting member and the transmission efficiency of optical signal is highly enhanced.
In the invention it is preferable that the light transmitting member is made of amorphous glass.
The light transmitting member of the invention is made of lead amorphous glass mainly containing a silicon oxide and a lead oxide or boro-silicate amorphous glass whose main components are boric acid and siliceous sand. In transmitting the light excited by the photosemiconductor element through the light transmitting member to the optical fiber member, the excited light is received by the optical fiber member as it is without being double-refracted in the light transmitting member. Accordingly the light can be efficiently received by the optical fiber member with the result that the transmission efficiency of optical signal is enhanced.
In the invention it is preferable that the light transmitting member is brazed to the fixing member via a metalized layer comprising a first layer made of at least one of titanium, titanium-tungsten and tantalum nitride, a second layer made of at least one of platinum, nickel and nickel-chrome and a third layer made of at least one of gold, platinum and copper.
According to the invention, the light transmitting member is fixed to the fixing member, for example, by coating the outer periphery of the light transmitting member with the metalized layer in advance and by brazing the metalized layer to the fixing member via a brazing material such as a gold-tin alloy. Thereby, the reliability of the attachment of the fixing member with the light transmitting member is enhanced, and perfect hermetic sealing of the attachment part of the housing for housing the photosemiconductor element is achieved and the photosemiconductor element housed within the housing may be operated normally and stably for a long period of time.
It is noted that the metalized layer is formed into a three-layer structure by laminating the first layer made of at least one of titanium, titanium-tungsten and tantalum nitride which are active to the amorphous glass and which bond strongly therewith, the third layer made of at least one of gold, platinum and copper which enhance the wettability of the brazing material in brazing the metalized layer to the fixing member and strongly attach the metalized layer to the fixing member, and the second layer made of at least one of platinum, nickel and nickel-chrome which prevents the bonding strength of the metalized layer to the transmitting member from decreasing due to diffusion of the first layer into the third layer by the heat of brazing. The metalized layer especially formed by laminating titanium-platinum-gold one after another is very suitable because of its high bonding strength to the light transmitting member and high wettability to the brazing material, and the metalized layer can be brazed strongly to the fixing member.
In the invention it is preferable that the fixing member is welded to an outer periphery around the through hole of the frame by a local heating method.
Since the fixing member is attached to the frame by welding by the local heating method and the heating range is narrow, thermal stress which occurs between the fixing member and the frame is very small even if the coefficients of thermal expansion of the two are different. As a result, an adverse effect of the thermal stress caused by the difference in coefficient of thermal expansion on the light transmitting member hardly occurs and the light excited by the photosemiconductor element which transmits through the light transmitting member can be prevented from being double-refracted in the light transmitting member and efficiently received by the optical fiber member, with the result that the transmission efficiency of optical signal is enhanced.
In the invention it is preferable that the local heating method is implemented by irradiating a laser beam.
Because the local heating by means of the irradiation of laser beam is implemented by YAG and carbon dioxide gas with power of a very high energy density of 104 to 107 W per 1 cm2 and the range to be heated is narrow, thermal stress generated between the fixing member and the frame is not so large and accordingly the light transmitting member is not adversely affected even when their coefficients of thermal expansion differ more or less.